Sprite generation by scripts doesn't check and overwrite if a script has been updated

[zephyris]

Currently output image files are not overwritten if the input images are older. However, it is not checked if the script which generates the output image files has been modified. This leads to the case that a script update won't lead to the output images being updated.

This can be manually worked-around, by manually deleting the output images for a particular script, deleting the repo and re-building the output, or deleting all pygen directories and output *8bpp.png, *bt32bpp.png and *rm32bpp.png etc. images. However, that's not a very stable solution.

Possibly solvable by adding a script self-check for date modified, probably solvable by switching to make.